The present invention refers to an easy-fit heat screening device for connecting a cooling pipe and a through-hole formed in a nozzle support ring of a gas turbine.
As is known, gas turbines are machines consisting of a compressor and single or multiple-stage turbine, where these components are connected together by a rotating shaft and where a combustion chamber is provided between the compressor and the turbine.
In these machines, air from the external environment is supplied to the compressor so as to pressurise it.
The pressurised air passes through a series of pre-mixing chambers which terminate in a converging portion and inside each of which an injector feeds the fuel which is mixed with the air so as to form an air/burning fuel mixture.
The fuel is introduced inside the combustion chamber and is ignited by means of suitable sparking plugs so as to produce combustion which is aimed at causing an increase in temperature and pressure and therefore enthalpy of the gas.
At the same time, the compressor supplies pressurised air which is made to pass both through the burners and through the casing of the combustion chamber so that the abovementioned pressurised air is available for fuelling the combustion.
Then the high-temperature and high-pressure gas reaches, via suitable ducts, the different stages of the turbine which converts the enthalpy of the gas into mechanical energy which is available to the user.
It is known, moreover, that in order to achieve the maximum efficiency of a given gas turbine it is necessary for the temperature of the gas to be as high as possible; however, the maximum temperature values which can be reached during use of the turbine are limited by the strength of the materials used.
Furthermore, in gas turbines, as in other turbine machines, it is necessary to prevent these hot gases from being drawn into spaces which are situated around the impellers of the said turbines.
It is therefore necessary to pressurise suitably the cavities which are adjacent to the fluid path so as to prevent a reduction in the efficiency and excessively high operating temperatures of the turbine impellers.
In the known art, cooling pipes are used for this purpose, said pipes conveying a cooling air which is supplied from the compressor and having, for example, to pass through the nozzles of the first low-pressure stage (which are thus provided with suitable holes) and through the nozzle support ring, in order to reach the zones to be pressurised.
In particular, the air which is tapped from the compressor has a temperature which is considerably lower than the operating temperature of the nozzle support ring. In order to avoid—in the zones where the cooling pipes pass through a through-hole formed in the nozzle support ring—the spread of high temperatures inside the said support ring, heat screening devices have been introduced.
These devices are arranged between the cooling pipe and the through-hole of the nozzle support ring and comprise a tubular structure.
It is pointed out, moreover, that these tubular structures also perform another function. In fact they act as a seat for the cooling pipe and ensure a sealed connection with the through-hole formed in the nozzle support ring.
The cooling pipe, which generally has spherical ends, departs from an external casing of the turbine and terminates inside the through-hole of the nozzle support ring.
In some types of gas turbine, the seat for the spherical end of the cooling pipe is provided by a bush inserted from the outside of the nozzle support ring and locked with a ring nut on the inside of the ring itself. This is possible when the through-holes passing through the nozzle support ring are straight, there being only one zone where pressurisation is to be performed.
In gas turbines, however, where there are two zones to be supplied, the through-holes passing through the nozzle support ring are of two types: one which is straight and the other inclined. In this case, in the prior art, two different types of fixing devices are used, one for straight holes and one for inclined holes, using also two different fixing methods.